For future reference...
The penultimate indicator holder for the mill is the "Zero-Set"
http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO=12387423&PMAKA=326-1124
IMNSHO it's the absolute best design since it's rigid, capable of holding the DTI in several ways, has a wide effective range and has a fine adjustment so you don't have to futz around tweaking the indicator stem to get the needle in the middle of the range.
Unfortunately, at ~$200 a pop, they're out of the "sensible" range for most folks, myself included. The options are to:
make your own - a lot of time for a seldom-used tool and an unrealistic task for a novice
just starting out
buy a cheap Chinese knock-off and fine tune it - what I did and works well. Not quite as smooth as the Cadillac model but that doesn't matter since the adjustment remains stationary after setting. It is, after all, merely a dongle to hold a DTI in a given location relative to the mill axis.
I can't immediately find a URL for the knock-off style but look around. They're out there.
Now, back to centering the RT.
First thing, as always in this trade, think about how accurately it must be centered for the job at hand. Will being off a couple of thou really matter? There are an awful lot of jobs out there where it won't. So let's consider some techniques that produce reasonable accuracy and don't even require a DTI.
Make a plug to fit the hole in your table. If the hole has a Morse taper, cut the tang end of an old MT drill or, alternately, buy an MT arbor - they're cheap. If the hole is cylindrical, simply turn a close-fitting plug. Drill and ream this plug 1/4" so a hardened alignment pin will fit into it. At the bottom of this hole drill and tap for something like an 8-32 screw. (More on the reason for this later.) Pop this plug in the RT hole.
Now, some ways to use this...
1. Put a 1/4" pin in the MM chuck/collet and fiddle the RT position until the pin slides freely into the hole in the plug.
2. Put your double-ended edge finder in the chuck/collet and seat the pointy end in the hole in the plug. Fiddle the RT until you can't feel any offset in the edge finder (note: mill not powered while doing this). Your fingers are incredibly sensitive and can detect much less than a thou offset. You can also use this option to fine tune a rough alignment done with #!.
3. Use the edge finder in conventional manner to perform the Osborne maneuver on the outer surface of the plug. (You can read about this maneuver on my website - OSBORNE.ZIP)
I prefer option #2 but any of these will work without a DTI. (I've made a lot of model engine parts on the RT and hardly ever used a DTI to align it.)
Nuances...
If the end of your plug sits below the RT table surface as mine does (rules out option #3), then you can make various custom pins to fit in the 1/4" hole and drop your part (that has a hole at the desired center of rotation) on the pin for automatic part centering on the table. The plug stays in place while you machine the part.
If your plug must be removed after centering and before mounting part to be machined, it may be difficult to get it out of the RT hole. (Especially so if the plug sits below flush as mine does.) This is where that 8-32 thread at the bottom of the hole becomes important. Fashion a miniature slide hammer from a length of 8-32 threaded rod and a chunk of heavyish scrap, thread into plug and pop that baby out without having to access the bottom of the table.
OK, maybe this is a bit more than Carl (or many of you other reader/lurkers) wanted to know (I've been accused more than once of providing "drinking from a fire hose" answers) but, remember that I'm trying to turn this thread into a sort of mini-course for the novice trying to build his first engine and learning stuff that will help him down the line even if he doesn't do it all right now. Remember, too much information is like too much cheese cake - it's just not possible.